Is they're anything we can do to improve keystoning? Maybe lining up several fresnels in a circular fashion? Or what about developing a special lens for it?

Improve how? My projector is at a pretty extreme angle and the keystoning still works perfectly.

Ok, an extreme angle? Is the whole thing in perfect focus? How come everyone else says only 15 degrees?

How does keystoning work anyway? People say that by placing they're projector at an angle they get a funy shapped picture, so you use keystone correction to fix it. If I use a normal lens and a piece of paper, hold it in front of a TV and try to project on an angle, it doesn't even focus! The middle is right but the top and bottom are either too close or too far. The only way I can get a trapazoid type of image /__\ << like that is by tilting the screen back. But the lens still has to be parralell to the screen and centered. But that doesn't exactly help to get the projector out of the way, it just makes a trapazoid.

And I would think that when you tilt the front fresnel for keystoning (however that works) you would have to reposition the triplet lens so the light is pointed straight at the lens. Seems to me you would loose light if the collector is not directing the light towards the triplet lens. And couldn't you use several fresnels lined up, each tilted a little more then the last to increase keystoning?

Someone needs to be edjucated.

Click for the education

http://www.lumenlab.com/forums/index.php?showtopic=92&st=0

John

(attempted to educate himself and failed)

I couldn't seem to find the answer to those questions there.

Not only that but I have two more questions:

The standard lens projects at a ratio of 1:1 meaning a 10 foot image is projected at 10 feet. But is that measuring the screen diagonally? If so is that for widescreen or full screen? If its for the lenght of each side, is it the horizontol or vertical measurement?

I assumed that it meant measuring diagonlly on a full screen. I am going to be using a 7 foot screen, so I figured 7 feet back. Now I figured that a 7 foot screen is about 4'2 high so the center is 2'1 or 25 inches below the top. Keystoning allows you to place the projector at 15 degrees away from the center. I figured that 15 degree keystoning allows you to raise the projector 3 inches for every foot. So being that the projector is 7 feet away from the screen, then I can raise it up 21 inches. So 25 inches minus 21 eqauls 4 inches. Therefore I can place the projector 4 inches below the top of the screen. Which isn't bad at all.

But then I realized that keystoning may and probably does effect the size of the image so these calculations are probably incorrect.

Mikau,
One thing you can do to help keystoning is tilt the screen a little also.

I went to see a movie at the theater the other day and noticed that the top of the screen leans out toward the crowd.

Thats fine if your looking up at it.

[QUOTE]Thats fine if your looking up at it.

I think I know what you are implying here and I don't think that is an issue. If it is help with keystoning you are looking for, Why not try some of the ideas before simply dismissing them.

They don't tilt the screen
based on where "You" are sitting. They tilt the screen based on the angle of the projector in relationship to the screen.

I'm new and this is one of the things that I can't get and idea how to do. I get the lense between the lcd and the pj lens needs to tilt, but don't know how to acomplish. How did you mount your lens so it can tilt? Remember I am an extreme newbie so explain in dummie terms. Read the link you posted and they are talking some complex ways - weighted lenses and all. Mine will be semi permenant so just need a simple keystoning plan.

I haven't gotten all of my parts yet but I plan on mounting the front fresnel to a metal rod at the top, hang it from the walls of the box, and let gravity do the rest.

Would like to here from people who have actually incorporated keystoning into their projectors and are now using it. What can be expected from the gravity keystoning. Is it acceptible to a certain degree tilt before it becomes rounded at the bottom, etc. I am looking at a ceiling mount and would like to have this knowledge. I plan on having no more than a 10 degree tilt. I also have a screen that I have made that can tilt out from the bottom about 5 degrees. THis slight tilt shouldn't be very noticable to the viewers. What can be expected from a 10 degree tilt with keystoning?

Works great for me.



Their are definite limitations. At high angles, the top and bottom are out of focus, top is darker, bottom is lighter (projecting upwards).

What angle is acceptable is a gray area. I'll take a stab and say under 10 degrees would be hard to notice.

- J

thanks jshire, always glad to hear from ya. Read many of your posts and have always been impressed, so I'm more comfortable that the angle I am going to be projecting at will give great results.